A subscriber's data circuit between channel bank and the customer premises carries fractional T1 bandwidth on a digital subscriber line (DSL) circuit. For example, for an ISDN rate digital subscriber line (IDSL), the circuit between the network termination at the customer premises and a D4 channel bank carries data on two combined B-channels. Such a circuit also carries a D-channel and an embedded operations channel (EOC), in normal ISDN fashion. A data service using such a line circuit, however, only transports the data (combined B-channels) through the network. The D-channel and the EOC are not carried through the network. To facilitate testing of the subscriber's circuit, the carrier operations and testing facilities transmit loop-back commands or the like in band to the channel bank. A command may be addressed to any active node along the subscriber's DSL circuit. The line card serving the subscriber in the channel bank detects the commands, removes the commands from the in-band data and reformats the commands for transmission over the EOC channel. The line card addresses the commands and sends the EOC format commands over the DSL circuit to the appropriate node along the subscriber's line. In response, the node connects the circuit in a loop-back mode, to enable test signal transmission and analysis.